The invention relates to a casing tube of a steering column of a motor vehicle and to a method for producing the same.
A casing tube of the generic type is disclosed in Australian Patent Document No. AU 29955 77 A. In this case, two tube sections are pushed one inside the other, with one end of one of the two tube sections being designed in such a manner that it bears frictionally against the inner or outer circumference of the other tube section. In the event of a crash, the tube sections are pushed further one inside the other, where they remain frictionally connected to each other over the entire displacement path. This results in particularly good absorption of the impact energy.
It is known from German Patent Document No. DE 23 35 708 A1 that two tube sections are pushed one inside the other in such a manner that only their facing ends form an overlapping region. In the pushed-in position achieved, the end of the inner tube section sits with a press fit in the end of the outer tube section, with the outer tube section following the press fit being widened in the manner of a step. In the event of a crash, such a high axial force acts on the steering gear that the press fit is eliminated and the inner tube section enters into the outer tube section without any resistance. This is intended to minimize the risk of injury to the driver should his head strike against the steering wheel. Although the known casing tube at least achieves a softer impact of the head or even avoids it, the casing tube does not make any contribution to the energy absorption which is necessary for preventing injuries to the occupants of the passenger cell, here in particular also in the driver's footwell.
The invention is based on the object of developing a generic casing tube and a production method for it to the effect that, in the case of a vehicle accident, in a simple manner firstly a hard impact of the driver against the steering wheel is prevented and secondly the greatest possible absorption of impact energy is achieved.
Owing to the frictional connection of the telescopic tube connections over their entire displacement path it is permitted firstly, that when the casing tube is subjected to an impact stress, as occurs in a vehicle crash, the tube sections are further pushed together and hence a reduction in the impact hardness for the driver is obtained, and secondly, the permanent frictional work in the relative displacement of the tube sections on each other causes kinetic energy, i.e., impact energy, to be converted to a considerable extent. The design of a functional casing tube of this type can be shown by the invention to be very simple. The starting products used can thus be tubes which can be produced with little complexity and inexpensively, have a rectilinear profile, for example are drawn or produced from a blank and, after shaping by rolling, are welded with a longitudinal seam and, in addition, are of identically shaped design and merely have to have different dimensions, so that they can be inserted one inside the other with play. The loose insertion one inside the other enables the tube sections to be positioned axially very exactly with respect to each other, so that after the frictional connection has been obtained, a casing tube which is virtually free of tolerances in its length is produced in a manner enabling it to be reproduced. The local, relative expansion of the inner tube section relative to the outer tube section in the inserted position of the two tube sections enables the two tube sections to be fixed frictionally on each other without great complexity. The expansion is sufficient for the two tube sections to be connected to each other by a form-fitting connection, as a result of which a particularly high absorption of the impact energy is obtained in a favourable manner. This can take place by thermal shrinking or magnetic pulse deformation of the outer tube section onto the inner tube section with its circumferential size being retained. Equally, the expansion of the inner tube section can be brought about while retaining the circumferential size of the outer tube section, for example by broaching. The frictional connection can be set in a specific manner by selection of the process parameters of the production methods, with it being possible, for the crash situation, for a certain axial displacement force to be established and thus for a uniform force profile to be brought about. With this selection, the force profile can be varied as desired according to requirement.